Radio-frequency amplifier



Filed NOV. 4, 1927 YINVENTOR ATTORNEY Patented Nov. 3, 1931 warren stares PATENT OFFICE. I

SYL'V'AN HARRIS, F PHILADELPHIA, PENNSYLVANIA, AS SIGNOR TO STEWART- I WARNER CORPORATION, OF CHICAGO, ILLINOIS, A CORPORATION OF VIRGINIA RADIO-FREQUENCY AMPLIFIER Application filed November 4, 1927. Serial No. 230,946.

My pending application, Serial No. 183,688

relates to a radio-frequency amplifier in which the net inductive reactance of each plate circuit is maintained at a value sufficiently low to prevent the generation of sustained oscillations at all frequencies which the circuit is designed to cover. That invention is intended primarily for use with radio receivers and its employment will result in a stable receiver, which will not produce the howls and whistles which cause so much an- 'noyance at the present time.

My present invention is an improvement upon this circuit with the object of preventing not only the production of sustained oscillations, but all regenerative eflect due to reaction between the plate and grid circuits.

It is well known that regeneration, while valuable for increasing the sensitivity of a radio receiver, affects the tone quality unfavorably. By my invention the tone quality of the receiver is improved.

It is also a well recognized fact that greater sensitivity and selectivity would be attainable if the primary and secondary windings of the coupling transformers between stages might have a high mutual inductance by being very closely coupled and by utilizing a primary winding of a large number of turns, possibly even as many turns as in the secondary winding and by using a greaternumber of radio-frequency stages. These desirable results are made possible by my inve11 tion.

Radio-frequency amplifiers at the present time employ coupling transformers in which the prnnary wlndlngs have very few turns and are loosely coupled with the secondaries,

although such a construction does not permit of an eflicient transfer of energy from one stage to the next, for the reason'that transformers of efficient design would introduce into the plate circuits such a high value of inductive reactance that the production of self sustained oscillations would inevitably result. As I have already stated my invention overcomes thisdifficulty.

In order that the invention may be better understood, attention is directed to the accompanying drawingin cwhich I show, diaof power.

grammatically, a radio-frequency amplifier which may be used with a detector and audiofrequency circuit of the usual design.

In this design, 1 and 2 represent the two radio-frequency tubes; An antenna 3 is shown connected by the coupling transformer 4 to the tuned grid circuit 5 of the tube 1. The plate circuit of tube 1 is coupled to the tuned grid circuit 6 of tube 2 by the coupling'transformer 7 The plate circuit of tube. 2 is coupled to the grid circuit of the detector tube (not shown)'by means of a similar coupling transformer 8. I

In the plate circuit of each radio-frequency tube, I introduce a variable condenser 9 in series with the primary winding of the trans former. These condensers are adjusted simultaneously with the tuning condensers 10 and 11 'in the input or grid circuits 5 and 6. Preferably all of the condensers are arranged so as to be controlled by a single dial, as indicated diagrammatically by dotted lines, which may be done in any suitable way, as

for example, by mounting all of the condensers on a single shaft.

By employmg a suitable capacltatlve re actance, such as a condenser 9 in series with the primary winding of the transformer,

coupling transformers which have high mutual inductance may be employed between the several radio-frequency stages and between the last radio-frequency stage and the detector ;-that is to say, their primary and secondary windings may be coupled together with extreme closeness, if desired, and the primary windings may have even as many turns asithe secondary. For example: the

the formula two windings may be wound together in the form of alternate turns upon the same form.

Of course, it is not necessary that the primary windings should have as many turns as the secondary or that the self-inductance of the primary should be equal to that of the secondary. It is only necessary that the productof the self-inductance of the primary Winding and the capacity of its associated condenser should be equal to the product of the self-inductance of the secondary winding and the capacity of its associated condenser or, in short, that the reactance of the plate circuit by itself, should be equal to the inductive reactance of the grid circuit by itself, which will necessarily result-in zero net reactance between thetwo circuits. This condition may be conveniently expressed by when L and L representrespectively the primary and secondary self inductances of the'transformer, C the capacity in the plate circuit and C the capacity in the grid circuit.

If the primary self-inductance is less than the secondary inductance, it is only necessary to make thecapacity of. the plate condenser relatively larger than the capacity of the condenser in the grid circuit in orderto establish the required condition. Therefore, by

the useof my invention, the coupling transformer may have the much desired high mutual inductance and the plate circuits a zero net inductive reactance, thus making possible a highly efficient receiver, without incurring the difficulties presented by regeneration.

In many radio receivers in which the radio frequency stages are coupled by the usual type of transformer having loose coupling between the'primary and secondary, an ap parent broadness of tuning is frequently noted, due to the fact that for any single adjustment of the circuits, the system as a whole will be resonantto two frequencies rather. close together. If the coupling is made even looser these two peaks may be made to approach one another until theo- I- have already demonstrated, to use coupling transformers in which the windings are very closely coupled. As a result the two resonance peaks may be made to occur at fre- .quencies so widely separated that only one of them is within therange of the receiver at .is varied, substantially as set forth.

any given setting. Thus the possibility of tuning in two stations simultaneously or the same station at two settings is eliminated, the selectivity of the receiver is thereby improved and the efficiency of the transformers greatly increased. Furthermore, the absence of all reactance effects between the plate and grid circuits makes feasible the employment of a greater number of radio-frequency stages than is possible at the present time, thusfurther increasing the sensitivity and selectivity of the receiver, without impairing its tone quality and stability- Having described my invention, what I claim as new is the'following:

1 A radio frequency amplifier comprising a three electrode audion associated with a tuned input clrcuit, means for tunlng sald circuit, an output or plate circuitcontaining inductance and a variable capacity, and means .for adjusting this capacity to maintain substantially in the output circuit a zero net inductive reactance, the plate capacity being continuously adjusted as the tuning capacity 2. A radio frequency amplifier comprising a three electrode audion, a tuned circuit or grid therefor, an output circuit containing inductance and a condenser and means for simultaneously adjusting the grid and plate condensers so that at all settings the condition L 0 =L 0 will be substantially maintained, where L L represent respectively the primary and secondary self-inductances .of the coupling transformer, C the capacity in the plate circuit, and C the capacity in the grid circuit. r

'3, A radio frequency amplifier comprising a three electrode audion, a tuned grid circuit therefor, means for tuning said circuit, a

plate circuit therefor containing inductance and capacity and means for varying said capacity so that regeneration due to the in-. ductive reactance of the plate circuit is substantially avoided, the plate capacity being continuously adjusted as the tuning capacity is varied. v

4. A radio frequency amplifier comprising a three electrode audion, a tuned input circuit therefor, means for tuning said circuit, an output or plate circuit containing inductance and capacity in series and means for varying said capacity to maintain the net inductive reactance of the output circuit at such value. that substantially no regenerative reaction is produced in the input circuit, the plate capacity being continuously adjusted as the tuning capacity is varied.

5. A multi-stage radio-frequency amplifier comprising a plurality of three electrode audions provided with tuned input circuits, each stage being coupled to the suuceeding stage by a transformer with such closely coupled windings of high mutual inductance that the resonance peaks are so widely separated that only one is within the range of the receiver at any given setting, avariable capacity in series with the primary winding of each transformer for maintaining the net inductive reactance of each plate circuit at'substantially zero, substantially as set forth.

6. A multi-stage radio-frequency amplifier comprising a plurality of three electrode audions provided with tuned input circuits, each stage being coupled to the succeeding stage by a transformer with such closely coupled windings of high mutual inductance that the resonance peaks are so widely separated that only one is within the range of the receiver at any given setting, a variable ca pacity in series with the primary winding of each transformer for maintaining the net inductive reactance of each plate circuit at substantially zero and a single dial control for simultaneously adjusting the series capacities with the tuning of the grid circuits; substantially as set forth.

SYLVAN HARRIS. 

